Drilling, the usual method of sampling, introduces some uncertainty.It is also rare that any information about the radiation from the burial soil can be obtained, as art objects are usually thoroughly cleaned. The age of the pottery, in principle, may then be determined by the relation Age = Accumulated dose / Dose per year Although conceptually straightforward, TL has proven to to be far from simple in practice. Should I be concerned about artificial irradiation? If the radioactivity of the pottery itself, and its surroundings, is measured, the dose rate, or annual increment of dose, may be computed.This is adequate for the purposes of authentication where the question is whether the piece was fired in antiquity or recently; it will not differentiate, say, between a classic Greek terra cotta and a Roman copy.In some categories of objects, from China, for example, the actual age is quite precisely known for short-lived styles, and it is possible to work "backwards" to get information about the environment in many parts of the world, and some other parameters not usually measurable for art objects.These will give an authentic date for a bogus object.It must be realized that TL dating is but one of the criteria for judging authenticity.
Among the reasons for this is the small amount of material that may be taken for testing.Using this information often reduces the uncertainty to 15-25 per cent. Nearly any mineral material which has been heated above 500C at a time one wishes to know is a candidate for TL dating. Porcelains, being nearly vitrified, are a special case requiring a fairly large solid core sample, and TL dating of intact objects is not recommended because of the damage caused by sampling.Most porcelain dating is done for insurance purposes on broken objects.It was employed in the 1950's as a method for radiation dose measurement, and soon was proposed for archaeological dating.By the mid-1960's, its validity as an absolute dating technique was established by workers at Oxford and Birmingham in England, Riso in Denmark, and at the University of Pennsylvania in the U. The Research Laboratory for Archaeology at Oxford, in particular, has played a major role in TL research.Much stoneware is not so hard as porcelain and may be sampled by drilling.The clay cores from lost wax metal castings may readily be tested. It is an absolute dating method, and does not depend on comparison with similar objects (as does obsidian hydration dating, for example). The thermoluminescence technique is the only physical means of determining the absolute age of pottery presently available. By comparing this light output with that produced by known doses of radiation, the amount of radiation absorbed by the material may be found. Most mineral materials, including the constituents of pottery, have the property of thermoluminescence (TL), where part of the energy from radioactive decay in and around the mineral is stored (in the form of trapped electrons) and later released as light upon strong heating (as the electrons are detrapped and combine with lattice ions).